The present invention relates to walking dragline excavators and is more particularly concerned with the frame structures of such excavators.
Large capacity walking dragline excavators have evolved into massive machinery structures having their own structural peculiarities. These machines are designed to handle large excavating loads, such as up to 350 cubic yards in each bucketload.
Until recently, walking dragline excavator manufacturers have failed to recognize, or have ignored the effects of torsional stresses on the frame structures of the machines, and the rail girders in particular. According to conventional practice, in order to prevent early failures of the rail girders, it has been the custom to increase the thickness of the steel plates forming the rail girders, and to provide closer spacing for the associated vertical web plates which are secured into a reinforcing grid to and between the deck and base plates of the horizontal frame structures of the machines. This has resulted in smaller cells within the frame structures, a larger number of cells, and has substantially increased the weight and fabrication costs of the frames.
By way of example, reference is made to Sankey U.S. Pat. No. 4,037,894, wherein it is stated that the joints between the lower rail pad and the vertical ribs are subjected principally only to compressive loads and, therefore, the rail pad is unattached to the vertical ribs. This implies that torsional loads and stresses are ignored or may not have been recognized.
In Kalve U.S. Pat. No. 4,329,795, it is recognized that tri-axial stresses, cyclical torsional and twisting forces imposed repeatedly in each operational cycle may eventually cause fatigue failure in the rail girder. The basic object of the disclosure in that patent is to provide efficient load stress relief by providing a separate rail beam which assures relative sliding movement between the rail beam flange and the underlying top plate. This sliding movement produces efficient load stress relief to the rail girder by redistributing some of the stresses to the lesser stressed parts of the frame. It does not add strength to the entire frame or the rail girder.
To a large extent, the present invention is a substantial improvement upon the disclosure in Kalve U.S. Pat. No. 4,611,440, in which it was recognized that the joints in the frame structures are subjected to torsional stresses, whether attached or unattached. Unattached joints are unable to transfer torsional shear stresses at the reentrant corners formed by the vertical webs of the rail beam and the rail pad, causing the vertical web plates and the rail pad member to distort independently of each other with a twisting and warping action under the torsional loading conditions. Without proper jointing, these plates provide only negligible torsional resistance to the frame, but are subjected to high torsional stresses individually.
In said U.S. Pat. No. 4,611,440, there is a showing of double rail beam web plates attached to the upper and lower rail pads, with vertical reinforcing I-shaped bars secured to and between the rail beam web plates at fairly widely spaced points. The purpose of this arrangement is to have both of the rail beam web plates and the rail pad in each instance act as a monolithic structural system. The structure is capable of transferring and distributing unbalanced loads from one web plate to the other web plate. However, a disadvantage has been encountered that in this particular system, due to limited space between the web plates of the rail beam which may only be 10" to 18", welding of the I-shaped bars must be effected from the outside of the rail beam through slots cut in the web plates of the beam. This is a costly and difficult welding process to attain proper securement of the I-shaped bar stiffeners.